An elliptical headlamp conventionally comprises a light source such as an incandescent filament or the luminescent arc of a discharge lamp, this source being placed in a first focal region of a mirror so that the light reflected by it is directed towards a second focal region situated in front of the first one. A lens, generally plano-convex, is focused on this second focal region, so as to project the light spot formed in said second focal region onto the road.
This light spot can be modeled, for example with a mask, to form a beam with cut-off as required, such as a dipped beam, an upper edge of this mask defining the profile of this cut-off.
Because of this possibility of forming a sharp cut-off, and because of the excellent recovery by the mirror of the light flux emitted by the source, such headlamps have been used successfully for many years to form dipped European beams with cut-off in a “V” shape.
In contrast, to produce a dipped beam in accordance with the standards in force in the United States of America, relying on a headlamp of the elliptical type poses more difficulty.
This is because one of the specific features of these standards lies in the requirement for a maximum brightness in the axis of the road which is substantially twice as high as in the European standard, whereas the standardized lamps used in the USA (for example of the 9006 standardized type) have a lower brightness (for identical overall flux and diameter) than the European lamps (for example of the standardized H7 or H9 type) since their filament is substantially lengthy.
Under these conditions, it can be understood that a headlamp possessing a conventional mirror in the shape of an axisymmetric ellipsoid will, in the absence of a mask, generate a beam exhibiting, in the vicinity of the optical axis, a central part of relatively great thickness, due to the images of the filament, which are elongated in the vertical direction and which are produced by the areas of the mirror which are situated immediately above and below the lamp, whereas regions of the mirror laterally very far from the lamp will generate lateral parts of the beam, corresponding to small, horizontally elongated, images of the filament, which will exhibit a substantially reduced thickness by comparison with said central part.
It is conventionally sought, however, in order to produce a satisfactory dipped beam, to give the beam, below the cut-off, a thickness which remains substantial over a significant extent in width, this being done particularly so as properly to illuminate the verges of the road.
It is also sought to obtain this result without degrading the concentration spot of the beam, which is directed in the axis of the road or slightly to the right (in the case of traffic driving on the right).